With the progress of the information technology field, there has been a demand to increase the capacities of optical memories such as CDs (compact discs), DVDs and BD (Blu-ray discs). By present optical recording methods, it is difficult to reduce the size of recording marks any further due to optical diffraction limit. In recent years, a great deal of attention is paid to an optical recording method using near-field light as technology for breaking through this diffraction limit. This near-field light is light locally generated near an aperture or fine particles of the size equal to or shorter than the wavelength of the light when the light is incident on these aperture or fine particles, and the spread of the near-field light does not depend on the wavelength of the incident light, but is determined by the size of an object on which the light is incident.
Conventionally, many methods for causing light to be incident on a sharp-pointed fiber probe or the like and generating near-field light at a micro-aperture formed at the leading end of the fiber probe or the like have been employed, but there has been a problem that light utilization efficiency in relation to incident light is poor. In recent years, a near-field light generating element utilizing a surface plasmon resonance of metal has been proposed as technology for drastically improving this light utilization efficiency (see, for example, patent literatures 1 and 2). These technologies are for inducing a surface plasmon resonance by irradiating a very small metal film with light having a suitable wavelength and generating near-field light near the metal film for recording and reproduction.
A conventional reproducing method as described above is described with reference to FIG. 17. As shown in FIG. 17, light 102 polarized in x direction is incident on a fan-shaped and electrically conductive scatterer 101 from a positive side toward a negative side in z direction. Here, a suitable wavelength is selected as the wavelength of the incident light, a surface plasmon resonance is induced in the scatterer 101 and near-field light is generated near an apex 103. An information recording medium 105 recorded with recording marks 104 is brought closer to a region of the scatterer 101 where the near-field light is generated, and is relatively moved while maintaining a constant distance. At this time, an intensity change of scattered light 106 is detected based on a difference in the relative positional relationship of the scatterer 101 and the recording marks 104, whereby information can be reproduced. Such a reproducing method is the one using the principle of so-called near field optical microscope.
What is problematic here is a S/N ratio at the time of reproduction. In other words, the scattered light 106 to become a reproduction signal is obtained by converting a part of the near-field light generated near the apex 103 of the scatterer 101 into propagation light by the information recording medium 105 and is difficult to efficiently introduce to a light receiving element. Thus, detectable light intensity is generally weak.
There is a method for increasing the intensity of the incident light 102 in order to increase the intensity of the scattered light 106. However, particularly in the case of using a rewritable material as a recording material of the information recording medium 105, there is a limit in increasing the intensity of the incident light 102 to prevent changes of the recording marks 104 by the near-field light at the time of reproduction.
In order to improve the S/N ratio, it has been also proposed to eliminate the influence of a part of the incident light 102 deviated from the scatterer 101 and directly reflected by the information recording medium 105, for example, by providing a light shielding film around the scatterer 101 or by providing a luminous body for emitting light having a wavelength different from that of the incident light 102 in a region of the scatterer 101 where near-field light is generated. Any of these methods is similar to the above in detecting a change of the propagation light resulting from the near-field light and cannot drastically increase signal intensity.    Patent Literature 1: Japanese Unexamined Patent Publication No. 2003-114184    Patent Literature 2: Japanese Unexamined Patent Publication No. 2003-149694